From the discovery of superconductivity in 1911 to the recent past, essentially all known superconducting materials were elemental metals (e.g., Hg, the first known superconductor) or metal alloys or intermetallic compounds (e.g., Nb.sub.3 Ge, probably the material with the highest transition temperature T.sub.c known prior to 1986).
Recently, superconductivity was discovered in a new class of materials, namely, metal oxides. See, for instance, J. G. Bednorz and K. A. Muller, Zeitschr. f. Physik B-Condensed Matter, Vol. 64, 189 (1986), which reports superconductivity in lanthanum barium copper oxide.
The above report stimulated worldwide research activity, which very quickly resulted in further significant progress. The progress has resulted, inter alia, to date in the discovery that compositions in the Y-Ba-Cu-O system can have superconductive transition temperatures T.sub.c above 77K, the boiling temperature of liquid N.sub.2 (see, for instance, M. K. Wu et al, Physical Review Letters, Vol. 58, Mar. 2, 1987, page 908; and P. H. Hor et al, ibid, page 911). Furthermore, it has resulted in the identification of the material phase that is responsible for the observed high temperature superconductivity, and in the discovery of composition and processing techniques that result in the formation of bulk samples of material that can be substantially single phase material and can have T.sub.c above 90K (see, for instance, R. J. Cava et al, Physical Review Letters, Vol. 58(16), pp. 1676-1679), incorporated herein by reference.
The excitement in the scientific and technical community that was created by the recent advances in superconductivity is at least in part due to the potentially immense technological impact of the availability of materials that are superconducting at temperatures that do not require refrigeration with expensive liquid He. Liquid nitrogen is generally considered to be one of the most advantageous cryogenic refrigerants, and attainment of superconductivity at or above liquid nitrogen temperature was a long-sought goal which until very recently appeared almost unreachable.
Although this goal has now been attained, there still exist barriers that have to be overcome before the new "ceramic" superconductors can be effectively utilized in technological applications. In particular, the ceramic high T.sub.c superconductive materials are relatively brittle. Development of techniques for fabricating the brittle compounds into bodies of desirable size and shape (e.g., wires or tape), and of techniques for improving the strength and/or other mechanical properties of ceramic superconductive bodies, is an urgent task for the technical community. Furthermore, techniques for increasing the critical current density J.sub.c of bodies formed from superconductive compounds are also of great significance.
For a general overview of some potential applications of superconductors see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDS and MACHINES, Plenum Press 1977; and S. Foner and B. B. Schwartz, editors, Superconductor Material Science, Metallurgy, Fabrications, and Applications, Plenum Press 1981. Among the applications are power transmission lines, rotating machinery, and superconductive magnets for, e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage, as well as junction devices and detectors. It is expected that many of the above and other applications of superconductivity would materially benefit if high T.sub.c superconductive material could be used instead of the previously considered relatively low T.sub.c materials.
The art has followed three approaches in producing ceramic superconductive compound bodies. One approach comprises providing the desired compound in powder form, producing a bulk body from the powder by any appropriate technique (e.g., cold or hot pressing in or through a die of desired size and shape, or forming a slurry and producing a tape therefrom by the doctor blade technique) and heat treating the resulting body. See U.S. patent application, Ser. No. 036,168, filed Apr. 6, 1987 for E. M. Gyorgy et al, titled "Apparatus Comprising a Ceramic Superconductive Body, and Method for Producing Such a Body now abandoned." The heat treatment invariably comprises treatment at a relatively high temperature that is intended to produce sintering of the powder particles, followed typically by optimization of the oxygen content of the material. The thus produced superconductive bodies typically are relatively porous (e.g., about 85% dense, depending on processing conditions). Furthermore, powder particles may not always be in intimate contact with their neighbors. The presence of voids and/or poor contact between particles is thought to be a possible reason for the relatively low strength and critical current of bodies produced from superconductive oxide powder by ceramic processing techniques.
A recently filed U.S. patent application Ser. No. 046,825, filed May 5, 1987 for S. Jin et al now abandoned) disclosed that some properties of superconductive compound bodies (e.g., their mechanical strength) can be improved by admixture of an appropriate metal powder (e.g., Ag) to the superconductive powder.
The second approach typically comprises forming a "preform" by introducing a quantity of superconductive compound powder into a tubular normal metal body, reducing the cross section of the preform by, e.g., drawing through a die (or dies) or rolling, until the desired wire or ribbon is produced. The wire or ribbon is then typically wound into a coil or other desired shape, followed by a sintering treatment and, possibly, an oxygen content-optimizing treatment. Two recent U.S. patent applications Ser. No. 036,160, filed Apr. 6, 1987 for S. Jin et al now abandoned, and Ser. No. 046,825, filed May 5, 1987 for S. Jin et al now abandoned) disclose techniques for forming metal-clad high T.sub.c superconductive bodies. Such bodies typically are also relatively porous, and have the relatively low T.sub.c associated with high T.sub.c superconductors produced by ceramic processing techniques.
The third approach to forming superconductive compound bodies comprises depositing a thin layer of the superconductive compound on an appropriate substrate. Deposition can be by any appropriate method, e.g., electron beam evaporation, sputtering, or molecular beam epitaxy. Another recently filed U.S. patent application Ser. No. 037,264, filed Apr. 10, 1987 for C. E. Rice now abandoned) discloses that thin superconductive films can be produced by forming a solution on a subtrate, and heat treating the thus formed thin layer. The high T.sub.c compound thin films known to the art are thought to be substantially 100% dense, and at least in isolated instances relatively high critical currents have been observed in such layers.
In view of the fact that technologically significant superconductive wires, ribbons, and other bodies have to be able to carry relatively high current densities and to be able to withstand relatively large forces, fabrication methods that can result in high T.sub.c superconductive bodies having improved properties (including higher J.sub.c and, typically, greater strength and thermal conductivity) would be of considerable significance. This application discloses such a method.